Solids refining apparatus

ABSTRACT

A process and an apparatus provide a purified material by employing a rate of condensation of the material which is substantially greater than the rate of solidification of the material. Telleurium and cadmium are effectively purified by the process.

The invention described herein was made in the course of work supportedby the Materials Processing in Space Program of the National Aeronauticsand Space Administration.

This is a continuation of copending application Ser. No. 630,503, filedon July 13, 1984, now U.S. Pat. No. 4,584,054.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of highly purified solids. Thisinvention more particularly relates to a vapor-liquid-solid (VLS)apparatus for preparing highly purified solids, such as tellurium andcadmium. This invention especially relates to an improved apparatus forpreparing highly purified solids of tellurium or cadmium by the combinedeffect of distillation and normal freezing wherein the rate ofcondensation is substantially greater than the rate of crystallizationso that the excess liquid continually washes the growing solid removingthe impurities rejected by solidification.

2. Description of the Prior Art

Vacuum distillation of metals is a well known process. See(Ultrapurification of Semiconductor Materials, Proc. Conf. onUltrapurification of Semicond. Mat., Boston, Mass., April 1961, M. S.Brooks and J. K. Kennedy, Eds. (MacMillan, New York 1962); Harman etal., J. Phys. Chem. Solids, 2 181 (1957)). The use of vapor transport tomove material into a process tube is also known (Lawson et al., 9 J.Phys. Chem. Solids, 325 (1959); Dziuba, 26 Acta Phys. Polonica, 897(1964)). Faktor and Garrett in "Growth of Crystals from the Vapour",(Chapman and Hall, London 1974) discussed the effect of small amounts ofinert gas on vapor transport. Russell and Woods (46 J. Crys. Growth 323(1979)) demonstrated that extraneous gas evolving from a sealed systemis difficult to control and requires clever manipulation for detectionand measurement. Some investigators who experienced low vapor flux in aVLS system assumed that the solid growth was limited by the processesoccurring at the liquid-solid interface. (Andryushenko et al., 15 Izv.Akad. Nauk SSSR, Neorg. Mater 1573 (1979).)

It is an object of this invention to prepare highly purified solids.

It is another object of this invention to improve VLS systems forpreparing purified solids.

It is a further object of this invention to prepare highly purifiedtellurium and cadmium in an improved VLS system.

The achievement of these and other objects will be apparent from thefollowing description of the subject invention.

SUMMARY OF THE INVENTION

Briefly, the objects of this invention are achieved by combiningsegregation by solidification with distillation producing a materialwith an impurity distribution similar to that of normal freezing. Inthis liquid-vapor-solid process vapor condenses into a liquid dropsubstantially faster than material in the drop solidifies and the excessliquid is returned to the source.

In this invention, an improved vapor-liquid, solid (VLS) solids refiningprocess is provided which comprises:

(a) establishing and maintaining in a closed VLS system having anevaporation zone and an condensing-solidifying zone an ambient gaspressure which is substantially below the vapor pressure at its meltingpoint of a material located in the evaporation zone, said material beingthe material to be purified,

(b) evaporating the material in the evaporation zone to provide vaporsof the material, said material having a vapor pressure in theevaporation zone greater than the vapor pressure of said material at itsmelting point,

(c) maintaining a temperature in the condensing-solidifying zoneeffective to condense the vapors of said material to a liquid and tosolidify said material at a rate substantially less than the rate ofcondensation of said material, whereby the liquid material continuouslywashes the surface of the solid material in the condensing-solidifyingzone, and

(d) transporting the unsolidified portion of the liquid material fromcondensing-solidifying zone to the evaporation zone.

In addition, this invention concerns an apparatus in which the aboveprocess may be practiced. This apparatus comprises

(a) an evaporator having enclosed sides, an enclosed lower end and anopen upper end provided with a ball joint,

(b) a process tube having enclosed sides, an enclosed upper end and anopen lower end provided with a ball joint, the ball joint of saidevaporator mating with the ball joint of said process tube to providecommunication between the interior portions of said evaporator and saidprocess tube,

(c) heating means effective to evaporate material located in theinterior of the lower end of the evaporator,

(d) cooling means effective to condense and solidify vaporous materiallocated in the interior of the upper end of the process tube,

(e) positioning means effective to position said heating means relativeto the longitudinal axis of the evaporator, and

(f) vacuum means effective to evacuate the interior portions of theevaporator and the process tube and to maintain a sub-atmosphericpressure outside of the mated evaporator and process tube which is lowerthan the pressure in the interior of the mated evaporator and processtube.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing of an apparatus in which the process of theinvention may be practiced.

FIG. 2 is a drawing of a spring collet which may be employed in theapparatus of FIG. 1.

FIGS. 3A, 3B and 3C are a schematic presentation of several of thestages which may be employed in the subject process.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention relates to improvements in providing materials inpurified form. Briefly, the material to be purified is evaporated in aclosed system which has been previously evacuated to high vacuum. Thevapors from the material pass to a cold area of the system where thematerial is condensed and then solidified at a rate substantially lessthan the rate of condensation. In this fashion a pendulus liquid dropforms on the solid material. The drop grows until it falls from thesolid material thereby continually washing the surface of the solidremoving any impurities that may be accumulating on the surface.

This invention may be utilized with a wide variety of materials toprovide these materials in highly purified form. The materials which maybe employed in this invention must have particular properties whichmakes them susceptible to the purification procedures employed. Thematerial must have a vapor pressure under the evaporation conditions inthe system which is greater that the vapor pressure of the material atits melting point. In general, the vapor pressure under the vaporizationconditions should be 10 to about 300 times, preferably about 50 to about100 times the vapor pressure of the material at its melting point.Examples of useful materials include members of Groups II B and VI A(except oxygen) of the Periodic Table, the rare earths, the alkalimetals, the alkaline earth metals and compounds which evaporatecongruently, such as Cds. Although vitreous silica has proven to be auseful material of construction for the apparatus of this invention,some of the materials to be purified, as for example, the alkali metals,require the use of a material of construction other than vitreous silicawhich would not be subject to attack by the material being purified. Ina particularly preferred embodiment, telleurium and cadmium can bepurified by the process of and in the apparatus of this invention. Theseare the materials which are the subject of the following detaileddiscussions.

The key element of this invention and the aspect of it which leads tothe high degree of purification obtained is the use of a level of heatremoval which causes the material to be solidified at a rate which issubstantially less than the rate of condensation of said material. Ingeneral, the solidification rate should be about 0.5 to about 10%,preferably about 1 to about 5%, of the condensation rate. Thus, as thesolid material is formed, a pendulus drop forms thereon. Periodically adrop of this liquid falls from the solid back into the evaporation zone.This continuous washing of the solid surface removes any impuritiesrejected during the solidification providing a highly purified product.

In a preferred embodiment of this invention, tellurium and cadmium aresubjected to the purification process. This VLS process and apparatusassures purified, largely single crystal Te and Cd loaded into a silicaampoule ready for synthesis and growth of CdTe or (Hg-Cd)Te. The systemalso provides vacuum prebakeout of the silica.

The source material, Te or Cd, is in an evaporator, vertically below andconnected to the ampoule by a ground silica ball joint. The evaporatortemperature maintains a vapor pressure of 10 torr (490° C. for Cd, 630°C. for Te), while a heat extractor cools the growing crystal at the topof the ampoule. Highly non-equilibrium condensation of the vaportransports about 10 mg/s into a pendulus liquid drop on the crystal face(321° C. for Cd, 450° C. for Te). The heater moves downward so that thecrystal grows at about 100 μg/s, 1% of the liquid accumulation rate. Theexcess liquid causes the drop to grow until part of it falls back to theevaporator, about twice each minute. This dropping completely mixes theliquid at the growth face, and also the source material. The growingcrystal is thus continually washed by freshly distilled liquid so thatimpurities rejected by solidification do not accumulate there. Theimpurity profile in the finished crystal is the ultimate distributionprofile for multiple zone passes, except that the effective segregationcoefficient is the product of those for solidification and vaporization.

The entire process may be described as a five step process: Step 1 - Theampoule is baked at 120° C. and 10⁻⁸ torr to remove surface moisture.Step 2 - The heater is lowered over the evaporator, a shutter interposedin the open ball joint, and the Te charge outgassed at a vapor pressureof 10⁻³ torr (329° C.). Step 3 - The shutter is removed, the jointclosed (without any grease, as leakage is outward) and the Te distilledinto the ampoule. Step 4 - The evaporator is exchanged for onecontaining Cd, which is also baked at 10⁻³ torr (219° C.) with theshutter in place. Step 5 - The Cd is distilled into the ampoule on topof the Te, which remains cool and solid.

The entire process is done in a vacuum bell jar with the pressuremaintained at or below 10⁻⁸ torr by an oil diffusion pump and LN₂ cooledchevron trap. At the end the bell jar is back filled with dry nitrogenso that the ampoule can be transferred to the sealing station withoutadmitting oxygen or moisture.

The following is an example of an embodiment of the present invention.

Apparatus

As constructed, the heart of the system comprised process tube 2 andevaporator 4 connected by a ground ball joint 6, all made of vitreoussilica, as shown in FIG. 1. Ball joint 6 was size 18/9 fromHeraeus-Amersil, ground and polished by hand first with 1.0 micron andthen 0.3 micron polishing alumina suspended in distilled water. Thedemountable joint was originally used so that process tube 2 could beweighed to determine accurately the mass of the charge loaded into it.No grease was used on the joint as it had to work at high temperature,and therefore the entire assembly was placed in a high vacuum bell jarso that leakage would be outward only. In fact, a few minutes polishingthe ball joint gave specular surfaces which gave no significant leakageeven of hydrogen over periods of weeks, as might be foreseen from thePoiseuille equation which states that the leakage is proportional to theproduct of the square of the passage cross section and the square of thepressure. Evaporator stem 8 housed thermocouple 10 for monitoring themelt temperature, and was mounted in a manually actuated lift system(not shown) which raised the evaporator to close the ball joint. Thelinkage allowed a maximum lift of 50 mm, and contained a spring whichassured positive closure pressure in spite of differential thermalexpansion between the metallic frame of the system and the vitreoussilica parts.

Process tube stem 12 was gripped by a spring collet as shown in FIG. 2,which in turn was mounted in a through hole in a copper tank filled withliquid nitrogen. The assembly was designed to extract the heats ofcondensation and solidification of the distillate by thermal conductionthrough the multiple berillium copper spring contacts of the collet.

As shown in FIG. 2, the spring collet or spring ring consists of a setof fingers or spring contacts which grip the process tube stem. Eachindividual spring contact applies a force, and the multiplicity ofspring contacts in the collet maintains a firm grip on the process tubestem.

Heater 14 was a platinum - 30% rhodium wire wound on a 30 mm i.d.vitreous silica tube 16 which was provided with bumps to keep thewinding properly spaced. The overall length of the winding was 356 mm,and the lowest 50 mm were wound at double density so as to provide aslight temperature gradient. There were two concentric silica tubes 18and 20 around the heater, and between them was reflector 22 made ofstainless steel foil, slotted from end to end on opposite sides toprovide windows for illuminating and observing the interior. Each endwas covered by a triple layer of stainless steel plate 24 and 26, whichserved as support and insulation. The end plates had 25 mm diameterholes for passage of evaporator 4 and process tube 2, and the hole inupper plate 24 could be closed by a manually operated stainless steelshutter (not shown). To attain a temperature of 630° C. required about250 watts.

The entire heater assembly could be raised or lowered by a set of threerecirculating ball lead screws chained together and driven by a steppingmotor which could be programmed to provide a fast transverse or crystalgrowth speed down to 3.33×10⁻⁴ mm/sec.

The vacuum system, as shown in FIGS. 3A, 3B and 3C was a bell jar with afeedthrough collar on a baseplate, pumped by a diffusion pump usingDC-705 fluid and a liquid nitrogen cooled chevron trap. The cold colletsupport for the process tube at the top of the chamber and theassociated plumbing constituted a very effective Meissner cryocoil,which when charged with liquid nitrogen permitted reaching a pressure of5×10⁻⁹ torr The heater movement, evaporator lift, and shutter actionwere all operated through Ferrofluidic rotary seals. Two ion gages and aquadrupole residual gas analyzer were used for monitoring the vacuum.Type E thermocouples were used to monitor the temperatures at theevaporator and the cold collet.

Operation

FIGS. 3A (Stage One), 3B (Stage Two) and 3C (Stage Three) showschematically the three stages associated with preparing a process tubeand loading the first element, which was tellurium. Prior to closing andevacuating the bell jar, the raw tellurium was loaded into theevaporator and the process tube was secured in the collet.

In the first stage (FIG. 3A) the system was evacuated, the heater wasmoved to its upper limit, and the evaporator lift was lowered to openthe ball joint. The evaporator was clear of the bottom of the heater andwas not greatly heated by it. With the collet cooled only by radiationand about 200 watts applied to the heater, the process tube was bakedover 550° C. The collet temperature stayed at 100° C., which was notenough to degrade the berillium copper grip springs. Since the onlycryogetter was the chevron trap in the pump, the vacuum was limited to10⁻⁷ torr, but these conditions were adequate to clean up the system andremove the surface water from the vitreous silica process tube in oneday.

In the second stage (FIG. 3B) the heater was moved to its lowestposition and the shutter was closed across the top, between the halvesof the open ball joint. The tellurium charge was roasted at 329° C.,where the vapor pressure was 10⁻³ torr. At this temperature, the loss oftellurium was negligible but surface oxygen was driven off and othervolatile contaminants including water were removed. Since the chamberpressure was low enough for molecular flow conditions to hold, theclosed shutter adequately protected the inside of the process tubesimply by blocking the line of sight path. After an hour, the telluriumcharge was effectively outgassed.

Distillation of the tellurium took place in the third stage (FIG. 3C).The ball joint was closed and the heater moved nearly to the top of itsrange so that the evaporator was just inside the lowest part. With 250watts the temperature was brought to 630° C., and the cold collet wasfed with liquid nitrogen. As the heater was stepped slowly downward, thetip of the process tube emerged from the top of the heater. When thetemperature of the tip dropped to 499° C., liquid which had condensedthere began to crystallize. The point of solidification remained nearlyfixed with respect to the heater, and as the heater moved slowlydownward, the crystal grew from the hanging drop, inverted Bridgmanstyle. The vapor pressure at the source was 10 torr, while that of thedrop at the melting temperature was only 0.16 torr, and so a vaportransport was established. The downward speed on the heater was adjustedso the rate of crystallization was a small fraction of the rate ofcondensation, with the result that the drop grew until the surfacetension could no longer support it, whereupon the major part fell backinto the evaporator. This was repeated periodically, stirring the melts.The first solid stuck firmly to the tip of the process tube, but beyondabout 5 mm growth proceeded in a fashion separated from the tube wall.When sufficient tellurium had been crystallized, the system was shutdown, the ball joint opened, and the bell jar back filled with drynitrogen.

The evaporator was then cleaned and reloaded with cadmium which was tobe transferred to the process tube containing tellurium. The secondstage process was repeated with temperature at 219° C. giving a cadmiumvapor pressure of 10⁻³ torr. Then the third stage was repeated with theevaporator at 490° C. (10 torr). Cadmium freezes at 321° C., at whichtemperature the vapor pressure of tellurium is less than 10⁻³ torr, andso it was possible to start growing a cadmium crystal directly on thetellurium. At first the tellurium surface blackened, apparently by theformation of a thin layer of passive cadmium telluride. Cadmium crystalgrowth was started at the same location in the heater where telluriumgrowth terminated. Since the heater had to be moved for the repeat ofthe second stage, the number of steps applied to the motor was countedso that an exact reversal could be programmed to return the heater toits former position.

It was observed that when commercial zone refined tellurium was firstmelted, a large and variable quantity of dissolved hydrogen wasreleased. Trapped in the process tube, this hydrogen had a profoundeffect of the transport of matter and heat, and also on the form of thegrown crystal. Specifically, the presence of a gas, such as hydrogen,significantly reduces the quantity of material transported from theevaporator to the process tube. It was found that this hydrogen could beremoved if the charge in the evaporator was resolidified and cooledbelow 329° C. At this temperature the ball joint was opened to dump thegas into the bell jar for analysis and removal without loss oftellurium. If this were not done, the hydrogen could remain in theprocess tube for weeks. Alternately, any uncondensable gases releasedduring the evaporation and which will adversely influence the rate oftransfer of vapors from the evaporator to the process tube could bevented by providing a small vent tube in the process tube. This venttube would carry the vented gas, together with a small portion oftellurium to a cold trap where the tellurium would be removed from thevacuum system.

Transport

Faktor and Garrett treated the effect of small amounts of inert gas onvapor transport and presented equations from which the reduction in thevapor transport in crystal growth system could be calculated.

The hydrogen found in the present VLS system was probably dissolved inthe tellurium bars when they were zone refined, as that process uses ahydrogen atmosphere. To evaluate the effect of hydrogen in the processof the present invention, consider first the case in which all of theinert gas has been vented and the process tube is pumped 10⁻⁸ torr orless. The evaporator, or source, is maintained at 10 torr, while thesink is maintained close to the melting temperature, where the vaporpressure is 0.16 torr for tellurium and 0.10 torr for cadmium. ThePoiseuille equation for viscous flow is ##EQU1## where F is mass flowrate, M is the molecular weight, a is the tube radius, l its length, Rthe gas constant, P₁ is the source pressure, and P₂ the sink pressure,here taken as negligible. The viscosity, η, is estimated from theSutherland equation for mercury and the fact that it is proportional tothe square root of the molecular weight.

    η(Cd, 763 K)=6×10.sup.-5 Pa s

    η(Te, 903 K)=10×10.sup.-5 Pa s.

The 10 torr source pressure is 1333 Pa, and effective values for a and lare 2.5 mm and 100 mm respectively in a typical arrangement. Thus

    F(Cd, 763 K)=40×10.sup.-3 g/s

    F(Te.sub.2, 903 K)=46×10.sup.-3 g/s.

The model assumes that the pressure increases as the square root of thedistance from the sink to the source, and that the temperature is nearlyconstant at the source value until very close to the sink, where thesudden drop in temperature and pressure make the condensation anonequilibrium process. Qualitative verification of these conditions ispossible: a sharp reduction of heater power caused condensation on thewalls of the process tube, uniform except for a very narrow band nearthe vapor sink.

If an inert gas is present, then the diffusion model of Stefan aspresented by Faktor and Garrett is used. An inert gas pressure very muchlower than the source vapor pressure has a strong effect of thetransport, as the inert gas is swept by the vapor toward the sink, whereit is compressed and can escape only by upstream diffusion. The totalpressure is the same at all points in the system, and the inert gasdominates at the sink. For tellurium, the mass flow, F', is ##EQU2##where D is the interdiffusion constant, P is the total pressure,constant and nearly equal to the source vapor pressure, and p is thepartial pressure of the component indicated at the place indicated.Since P>>p (Te₂, sink), and P-p (Te₂, source)=p (H₂, source), we canapproximate ##EQU3## For tellurium diffusing in hydrogen at 10 torr and903 K the diffusion constant is approximately 4×10⁻² m² /s. Assuming thesame tube dimensions as above and P=p (Te₂, source) at 903 K, this gives

    F'[p(H.sub.2, source)=1 torr]=0.8×10.sup.-3 g/s

    F'[p(H.sub.2, source)=10.sup.-1 torr]=1.6×10.sup.-3 g/s

    F'[p(H.sub.2, source)=10.sup.-2 torr]=2.4×10.sup.-3 g/s.

F' is orders of magnitude lower than F.

The heat of condensation is the product of the vaporization enthalphyand the vapor mass flow. The vaporization enthalphy for tellurium is1.28 j/g and that for cadmium is 1.00 j/g, and so the heat transportedby the vapor is from 1 to 50 mW for flow from diffusion controlled tothe viscous limit.

The solidification heat depends on the crystal growth rate and diameter.A typical process tube has an 8 mm inside diameter and so contains 0.30g of tellurium or 0.43 g of cadmium per millimeter of length. At atypical growth rate of 5×10⁻⁴ mm/s, the tellurium solidifies at 0.15mg/s and the cadmium at 0.22 mg/s. Since the fusion enthalpies arerespectively 0.137 and 0.054 j/g, the heat of freezing is 20 μw fortellurium and 12 μw for cadmium. This heat is negligible, and, further,in the present process, most of the vapor which condenses does notsolidify but drops back into the evaporator as liquid.

The thermal resistance between the growing crystal face and the coldcollet is composed of the process tube stem in series with the parallelcombination of the crystal and the process tube wall. Using 0.03 w/cmdeg for the phonon thermal conductivity of the silica process tube stem,the stem resistance is 530 deg/w, while that of the crystal and processtube varies from zero at the start of growth to about 150 deg/w when 10cm of tellurium has been grown. The thermal resistance of an additionalcadmium crystal is negligible. Since the temperature difference fromtellurium freeze to cold collet is 645 degrees, and from cadmium growthis 516 degrees, the heat conduction to the cold collet varies from 1.2 wat the start of tellurium growth to 0.76 w during cadmium growth. Thisheat is much more than that supplied by the vapor condensation alone,and so there must be significant radiative transfer from the heater tothe crystal. For this reason, crystal growth takes place about acentimeter down inside the heater.

The location of the crystal growth is self stabilized, in that thecrystal will advance more rapidly than the furnace travel until itcatches up with the right conditions for growth, or conversely, if thecrystal is too long, it will melt back. If growth is begun beforeventing the trapped gas in the tube, and subsequent to venting the samepower is applied at the same heater position, there is a drop oftemperature in the evaporator of as much as 20° C., and at the sametime, the growth face will retreat up the heater by up to 1 cm. This isdue to the increased heat pipe effect of the improved vapor flow.

Segregation and Purification

The vaporization and solidification are equilibrium processes, while thecondensation is not. Thus the segregation constant for condensation isclose to unity, and the composition of the hanging drop will be that ofthe vapor. If the source composition is C_(s), the vapor compositionC_(v), the drop composition C_(d) and the crystal composition is C_(c),then ##EQU4## define the various segregation constants, and it is clearthat the appropriate segregation constant for the whole process isk=k_(v) k_(f).

The hanging drop grows and falls back into the evaporator when itreaches a mass of about 200 mg, judged by the observed 4 mm diameter ofthe falling drop. This process is repeated at rates of from severaltimes per minute to less than once a minute, depending on the amount ofinert gas present. The departing drop agitates the remaining liquid andthe impact at the evaporator surface makes substantial waves, and sothere is complete mixing in each of the fluid phases. The resultingimpurity distribution is given by Pfann's equation for normal freezingwith ideal fluid mixing

    C=kC.sub.o (1-g).sup.k-1

where k=k_(v) k_(f) and g is the fraction of the original meltcrystallized. If the heater is turned off before the evaporator isempty, this results in a considerable purification, depending on thevalues of k_(v) and k_(f) for the particular impurity. If commercialzone refined tellurium is evaporated completely, there is generally aninert sludge remaining, whose mass is from 10⁻³ to 10⁻⁴ times theoriginal mass of tellurium, and whose composition is quite variable.

Finally, the crystal grown in this system is free of dissolved gas. Thismay be particularly important in the case of hydrogen in tellurium to beused in sealed silica ampoule growth of (Hg-Cd)Te, as in that case freehydrogen may reduce some of the silica, and the resulting water promotesticking of the crystal to the ampoule wall.

What is claimed is:
 1. A VLS apparatus for use in a solids refiningprocess which comprises:(a) an evaporator having enclosed sides, anenclosed lower end and an open upper end provided with a ball joint, (b)a process tube having enclosed sides, an enclosed upper end and a openlower end provided with a ball joint, the ball joint of said evaporatormating with the ball joint of said process tube to provide communicationbetween the interior of portions of said evaporator and said processtube, (c) heating means effective to evaporate material located in theinterior of the lower end of the evaporator, (d) cooling means effectiveto condense and solidify vaporous material located in the interior ofthe upper end of the process tube, (e) positioning means effective toposition said heating means relative to the longitudinal axis of theevaporator, and (f) vacuum means effective to evacuate the interiorportions of the evaporator and the process tube and to maintain asub-atmospheric pressure outside of the mated evaporator and processtube which is lower than the pressure in the interior of the matedevaporator and process tube.
 2. An apparatus according to claim 1wherein the evaporator and the process tube are composed of vitreoussilica.